1. Field of the Invention
This invention relates generally to coating removal devices and, more specifically, to an apparatus and method for high frequency induction heating for the removal of nonmetallic coatings from metal surfaces.
2. Background of the Invention
The performance and reliability of high technology ice breaker coatings on ships and drill rigs have proven inconsistent and unpredictable. Thus, major coating repairs are often necessary. To repair a coating, it is necessary to strip off all of the old coating down to the bare metal surface and recoat a large continuous area of the surface, as opposed to recoating small bare spots on the metal surface.
Conventional methods of repairing coatings have been found to be uneconomical. Thus, repair operations have been limited in scope to spot repairs and overcoating, neither of which provide a lasting solution to coating failures.
One method of spot repairing old coatings is grit blasting. However, grit blasting provides poor stripping rates, typically around 10-20 minutes per square foot. This is due to the fact that aged and failing ice breaker coatings still retain strong bond strengths of 500-1000 psi (pounds per square inch) and are thick and inherently resilient. Another problem with grit blasting deals with the logistics of the process. In order to grit blast a metal surface, a large quantity of special grit has to be shipped to the location of the metal surface. Furthermore, once the coating has been removed from the metal surface, the used grit must be properly disposed.
Recently, ultra-pressure water blasting has been used instead of grit blasting. Many of the problems associated with grit blasting are not a factor with ultra-pressure water blasting. It has been found that water blasting has a stripping rate of approximately 3-5 minutes per square foot which is up to three times faster than grit blasting. Inhibitors can be added to the water to prevent "rust bloom" from forming on the newly cleaned metal surface. Furthermore, water blasting provides a cleaner metal surface than grit blasting, and the problems of grit transportation and disposal are obviated under this method. However, ultra-pressure water blasting still does not have a high enough stripping rate to be economically feasible for large scale stripping of old coatings.
It has been discovered that heating a coating to about 200.degree. C. completely destroys the bond of the coating to metal surfaces for most organic coatings. However, the use of radiant and convective heat to remove coatings from thick metal surfaces proved problematic since the heat transfer through the coating is poor. The poor heat transfer causes the coating to become charred thereby releasing noxious gases. Furthermore, the use of radiant or convective heating provide inefficient stripping rates for coatings bonded to thick metal surfaces.
Therefore, a need existed to provide an improved method and apparatus for removing coatings from metal surfaces. The improved apparatus and method will use induction heating to remove the coating from the metal surfaces. The improved apparatus and method must also have a high enough stripping rate to be economically feasible for large scale stripping of old coatings and must not have the logistic problems associated with current stripping apparatuses and methods. The improved apparatus will be able to be manually moved across a metal surface. The improved apparatus can also be coupled to a remotely-operated utility vehicle which magnetically attaches itself to metal surfaces and is able to move about on the metal surface with high levels of precision and mobility while carrying the improved stripping apparatus as well as other types of equipment and payloads.